In view of recent scientific advances it seems more probable than ever that many hereditary diseases may eventually be cured by gene therapy; however, many problems still remain. One of these is that most of the DNA taken up by recipient cells is eventually degraded by intracellular nucleases. In the proposed study the possibility of using unusually modified DNA instead of normal DNA as donor genes will be explored in a model system. It is hypothesized that highly modified donor DNA will survive longer in an animal cell than would normal DNA. This study will investigate the uptake by animal tissue culture cells and the intracellular fate of different types of highly modified DNA from phage (T4, T6, SP-15, SP01, PBS1, and XP-12), three of which have already been shown to be much more resistant to purified nucleases than is normal DNA. Each of these contains a different pyrimidine substituting completely or largely for either thymine or cytosine. The extent of uptake, secondary structure, molecular weight and biological activity after uptake, and the persistance of these highly modified donor DNAs in the host cells will be compared to that of normal DNA and of 5-bromouracil-substituted DNA> Various methods to facilitate uptake will be compared. Penetration of macromolecular donor DNA into the cells will be carefully distinguished from binding to the cell surface. Analysis of uptake will be facilitated by several properties possessed by all or some of these DNAs, namely, presence of unusual pyrimidines, lower molecular weight than host DNA, separability of the two complementary strands, transfecting activity demonstrable in their bacterial hosts, and unusually high buoyant density in CsCl gradients.